Christchurch:Four people were arrested on Friday after two gunmen opened fire at two mosques in New Zealand's Christchurch city, with the police saying that there were "multiple fatalities".

Police Commissioner Mike Bush said the four arrested people comprised three men and one woman, CNN reported.

"We are not aware of other people, but we cannot assume there are not others at large," Bush told the media here.

The shootings took place at the Al Noor Mosque and Linwood Masjid, The New Zealand Herald reported.

One gunman, believed to be Australian, filmed as he shot victims in the mosque - and wrote a manifesto declaring his intentions, saying that "it is a terrorist attack", according to Bush.

He said that there were also multiple improvised explosive devices attached to vehicles as part of the attack.

"This goes to the seriousness of the situation," Bush said, adding that the police were not assuming the situation was limited to Christchurch at this stage.

Authorities advised people not to go to mosques until further notice. All Christchurch schools are on lockdown.

Resident Robert Weatherhead told Newstalk, that he took in people who escaped from the Al Noor Mosque.

He described the gunman as "white, aged in his 30s or 40s and wearing a uniform", but he had not been able to ascertain what the uniform was.

Another witness, who was in the front row of prayers when the gunman came in, told The New Zealand Herald that the suspect first shot people outside, adding that he heard the gun being reloaded about three times.

"The gunman began shooting in all directions," the witness said.

A woman told the Christchurch Star she lay in her car near the mosque as four to five men came running towards her.

Addressing a media conference in New Plymouth, Prime Minister Jacinda Ardern described the events in Christchurch as "one of New Zealand's darkest days".

Ardern described the event as "extreme and unprecedented violence".

Apart from numerous witness reports of casualties, a reporter following the Bangladesh cricket team, who are currently in New Zealand, tweeted that the team had "escaped from a mosque near Hagley Park where there were active shooters".

Player Tamim Iqbal tweeted that the "entire team got saved from active shooters".

Bangladesh Cricket Board spokesman Jalal Yunus said most of the team had gone to mosque by bus and were about to go inside when the incident took place.